Hard bubbles and hard wall segments will transform into normal ones, if an in-plane field is larger than a threshold value Hin. We measured the Hin values for some as-grown magnetic bubble materials. As d.c, magnetic bias field perpendiculer to the film increases, Hin decreases. It has been shown that the transformation from hard bubbles and hard wall segments into normal ones is a relaxation proccess which may last for a few seconds or more. The above results were explained qualitatively in terms of the concept of field-nucleated Bloch points suggested by Slonczewski and Maloze-moff.